Method to produce sintering powder by grinding process with carbon nano tube

ABSTRACT

Disclosed herein is a method of producing high-quality sintering powder by grinding metal powder along with carbon nanotube (CNT) particles. 
     More particularly, the present invention relates to a method of producing high-quality sintered compact by sintering well-dispersed powder prepared using advanced CNT composite to prevent cohesion of particles.

TECHNICAL FIELD

The present invention relates to a method of producing high-qualitysintering powder by grinding metal powder mixed with carbon nanotube(CNT) particles.

More particularly, the present invention relates to a method ofproducing a high-quality sintered compact by sintering well-dispersedpowder prepared using a CNT composite to prevent cohesion of particles.

BACKGROUND ART

Sintering is a process of forming a porous solid material or a compositematerial of two materials (for example, metals and ceramics) that do notmix with each other in a molten state.

During sintering, solid particles are placed in a mold and compressed toprovide a preform having a predetermined hardness by means of a press.Then, when the preform is heated to a temperature close to the meltingpoint of the material, a sintered compact can be formed as the particlesare fused together or some parts of them are coagulated. Metal productscan be produced by sintering. This process was first applied totungsten, which has a high melting point and is difficult to melt.

Recently, sintering is commonly used for various metals. In aconventional sintering process, a raw material is ground to have a smallparticle size and is then sintered. In this case, chemical grinding aidscan be used to prevent cohesion between particles. However, there hasbeen reported no case of using CNT to prevent the cohesion of particlesand obtain well-dispersed powder and sintering the powder to producehigh-quality products.

In the conventional sintering process, dispersion of metal (for example,aluminum) particles is often interfered with by cohesion between theparticles.

Further, the cohesion of particles often leads to decreased density andlarge pore size.

DISCLOSURE Technical Problem

Therefore, the present invention has been made in view of the aboveproblems and it is an object of the present invention to provide amethod of producing a high-quality sintered product with well-dispersedpowder.

It is another object of the present invention to provide a method ofproducing powder in an optimum dispersion state by grinding a mixtureprepared with CNT.

Technical Solution

In accordance with an aspect of the present invention, a method ofproducing sintering powder comprises adding a predetermined amount ofcarbon nanotube (CNT) to a raw powder material, and mixing the CNT withthe raw powder material and grinding the mixture.

Preferably, the method further comprises dispersing the CNT in ethanolbefore adding the CNT to the raw powder material.

Preferably, 2 wt % of CNT is added to 98 wt % of the raw powdermaterial.

Preferably, the raw powder material is aluminum powder and the aluminumpowder is sintered at 550° C.

As will be described in Examples described below, the CNT acts as notonly a grinding aid but also a dispersant, and prevents cohesion ofaluminum particles. When aluminum is ground alone, the particles tend tocoagulate into spheres. However, when CNT is added, only the particlesize of the powder material is reduced without changing particle shapeor other characteristics thereof. Thus, the present invention presents anew application of CNT as a grinding aid which reduces only the particlesize of aluminum without changing other characteristics thereof. Thatis, particles useful for the manufacture of nano-composites can beprepared without using another grinding aid during grinding ordispersing, since the CNT functions as the grinding aid.

ADVANTAGEOUS EFFECTS

The method of producing sintering powder according to the presentinvention enables the production of well-dispersed aluminum powder bymixing aluminum particles with CNT powder and grinding the mixture.

Desired dispersion of particles can be obtained by preventing cohesionof the powder during the sintering of aluminum powder.

Further, the problem of pore size increase caused by cohesion of theparticles during sintering of aluminum powder can be solved.

In addition, although aluminum particles tend to coagulate into sphereswhen the aluminum particles are ground alone using a ball mill, CNTallows only the particle size of the raw powder material to be reducedwithout significantly changing particle shape or other characteristicsthereof. Thus, the present invention proposes a new application of CNTas a grinding aid which reduces only the particle size of aluminumwithout changing other characteristics thereof. That is, since the CNTserves as a suitable grinding aid, particles useful for the manufactureof nanocomposites can be prepared without other grinding aids duringgrinding or dispersing.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart illustrating a method of producing a high-qualitysintered compact according to one embodiment of the present invention;

FIGS. 2 a and 2 b are SEM micrographs of aluminum and CNT used forexperiment;

FIGS. 3 a to 3 f are SEM micrographs of aluminum powder ground atdifferent times, in which FIGS. 3 a to 3 f correspond to 12 h, 18 h, 24h, 36 h, 48 h, and 72 h, respectively;

FIGS. 4 a to 4 f are SEM micrographs of CNT-Al powder ground atdifferent times, in which FIGS. 4 a to 4 f correspond to 12 h, 18 h, 24h, 36 h, 48 h, and 72 h, respectively;

FIGS. 5 a to 5 d are graphs depicting particle size frequencydistributions (FIGS. 5 a and 5 b) and cumulative particle sizedistributions (FIGS. 5 c and 5 d) of aluminum powder and Al-CNT powderin relation to grinding times;

FIGS. 6 a and 6 b are graphs depicting change of density and porosity ofa sintered aluminum compact and a sintered Al-CNT compact; and

FIG. 7 is a graph depicting hardness change of a sintered aluminumcompact and a sintered Al-CNT compact.

BEST MODE

Research into CNT reinforced composites is actively under way as a mainapplication of CNT in many countries, including the U.S. and Europe.

Research into CNT reinforced composites is mainly focused on CNTdispersal, CNT orientation, CNT/polymer interface control, CNT compositehigh-dimensional structuralization, and the like. These techniquesencompass those necessary for the long-term objective of realizing anultra-high-strength composite, from basic materials, toreinforcement/matrix composites, to reinforcement/matrix interfacialbindings, to orientation in reinforcement/matrix composites, etc.

As a matrix material for CNT, polymer-based materials are widely used.Meanwhile, it is expected that composite materials with improvedstrength, toughness, wear resistance, creep resistance, and the like maybe obtained by dispersing CNT in ceramics or metallic materials.However, research in this area is still insufficient.

As research continues, ceramics or metal-matrix nanocomposites will beused more widely than polymer-based matrices, particularly in theaerospace industry, because of their superior heat resistance and wearresistance. Whereas research into clay-dispersed composites is led bycorporations, those on polymer-based CNT-dispersed nanocomposites areactively carried out by universities, including Georgia Tech, RiceUniversity, Pennsylvania State University, University of Cambridge,Northwestern University and University of Delaware, and researchinstitutes such as NASA.

Major research accomplishments showed that addition of about 2-8 wt % ofCNT to a polymer matrix resulted in about 200% improvement of tensilestrength, about 350% improvement of rigidity, and about 60% improvementof hardness.

In Korea, research is generally focused on functional rather thanstructural nanocomposites. Research on CNT dispersed composites is alsocarried out with regard to the development of CNT/polymer composites fordisplays based on the electrical properties of CNTs. As yet, significantresearch on high-strength, ultralight CNT reinforced composites forstructural purposes has not yet entered manufacturing, design,simulation, or analysis.

The development of functional nanocomposites has been designated as abasic industry in a petrochemical organic new material division fornext-generation industrial growth in Korea, and, as such, research onthe development of functional nanocomposites is expected to progressrapidly. However, research on nanocomposites for structural purposes isrelatively lacking, considering the scale of Korea's automobile industryor the level of the Korean aerospace industry.

Research on nanocomposites carried out thus far has been led by chemicalengineers and is mainly focused on manufacturing techniques such assynthesis and manufacture of reinforcements for polymer matrixcomposites, fusion of reinforcements in matrices through dispersion,molding of final composites, and the like.

Further, the applications of nanocomposites have also been preferred forthe fields of electricity, electronics, optics, chemistry, chemicalengineering, etc. In additions, evaluation of physical properties ofcomposites has also focused on electrical, optical and chemical onesrather on mechanical behavior and structural performance. In the earlystages of the development of nanocomposites, research on theplausibility of manufacture and physical properties is very important.

For more systematic and effective R&D activities with regard to themanufacture of structural composites having desirable properties usingvarious matrices and reinforcing materials, establishment of atheoretical framework will be necessary.

However, not just inside Korea but also abroad, research on theprediction and evaluation of physical properties or structural behaviorsof nanocomposites is not sufficient. Such research is just beginning inmany countries. Later, active research will be required for thedevelopment and application of nanocomposites.

MODE FOR INVENTION

Preferred embodiments of the present invention will hereinafter bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a flow chart illustrating a method of producing a high-qualitysintered compact according to one embodiment of the present invention.Composition of source materials and manufacture process according to thepresent invention will be described in detail with reference to FIG. 1.Conditions for grinding experiments are given in Table 1.

In order to evaluate the role of CNT as a grinding aid, grindingbehavior of aluminum powder was observed while grinding a mixture ofMulti-Wall CNT (MWCNT) and aluminum powder.

FIGS. 2 a and 2 b are SEM (scanning electron microscope) micrographs ofaluminum powder (×5050) and MWCNT (diameter=about 20 nm, length=about 5)used in the experiment (FIG. 2 a: pure Al powder, FIG. 2 b: CNT). Acommonly used ball mill was used for the grinding experiment.

Experimental conditions are given in Table 1.

TABLE 1 Experimental conditions Item Experimental conditions n (rpm) 200d_(B) (mm) 10 Ball filling ratio ( - ) 0.3 Sample filling ratio ( - )0.05 Material of media Steel Temperature Room temperature Atmosphere Air

2 wt % of CNT (0.16 g) was dispersed in ethanol using an ultrasonicator.Then, 98 wt % of aluminum powder (8.15 g) was mixed with the CNT-ethanolsolution.

Subsequently, the two components were completely mixed by stirring for30 minutes using a stirring rod. Then, after evaporating ethanol bydrying at 50° C., grinding experimentation was carried out.

The ball mill used for grinding was equipped with a grinding jar(diameter=40 mm, height=55 mm) The weight ratio of the balls to thematerial was 10:1. The rotating speed of the ball mill was fixed at 200rpm, and the grinding time was varied from 12 hours to 72 hours.

Particle size distribution of the ground particles was measured usingMastersizer (Malvern Instruments, UK), and SEM micrographs were taken(JSM-5610, JEOL, Japan) for analysis of particle shape and dispersionstatus.

Sintering was performed using a plasma activated sintering (PAS)apparatus under the condition of 30 MPa and 550° C. The sinteringtemperature (550° C.) was determined from a preliminary experiment andwas identified as the optimum temperature at which the composite can beprepared without forming the compound Al₄C₃. Density and porosity weremeasured using a density meter based on Archimedes' principle.Mechanical property (hardness) was measured using a micro-Vickershardness meter.

FIGS. 3 a to 3 f are SEM micrographs of ground aluminum at differenttimes (12 h, 18 h, 24 h, 36 h, 48 h, 72 h). Particle shapes anddistribution states of the ground particles can be compared to eachother from FIGS. 3 a to 3 f. When pure aluminum powder was groundwithout adding CNT, the originally flat particles became spherical onesat 12 hours. At 48 hours, the particles began to coagulate, forminglumps.

Interestingly, as the grinding time increased, some of the coagulatedparticles were ground again. Further, the particle surface becamerougher as the grinding continued. FIGS. 4 a to 4 f are SEM micrographsof ground CNT-Al at different times (12 h, 18 h, 24 h, 36 h, 48 h, 72h).

As seen from FIGS. 4 a to 4 f, cohesion of particles was not apparentwhen CNT was added. At an earlier stage of the grinding, particle sizedecreased little by little, and their original status was maintained(FIG. 4 a). Cohesion was not so apparent as when CNT was not added, evenat longer times (FIGS. 4 b to 4 d). At 48 hours, grinding proceeded nofurther as equilibrium was reached, but cohesion was not observed.Therefore, it can be seen that CNT prevented cohesion of aluminumparticles, acting as grinding aid and dispersant.

The ground particles were coagulated into spherical particles whenaluminum was ground alone, whereas only the particle size decreasedwithout change in particle shape when CNT was added. Thus, the presentinvention proposes a new application of CNT as a grinding aid whichreduces only the particle size of aluminum without changing othercharacteristics thereof. That is, particles useful for the manufactureof nanocomposites can be prepared without using another grinding aidduring grinding or dispersing, since the CNT functions as a grindingaid.

FIGS. 5 a to 5 d are graph depicting particle size frequencydistributions (FIGS. 5 a and 5 b) and cumulative particle sizedistributions (FIGS. 5 c and 5 d) of aluminum powder and Al-CNT powderin relation to grinding times.

As described above, when CNT was used as a grinding aid, the particlesize was consistently decreased even after 48 hours because cohesion didnot occur. This is because the CNT particles were mixed with the powdermaterials as described above, preventing cohesion thereof. Further, theyadequately prevented cohesion of the balls with the powder materials andwith the inner wall of the grinding jar.

Generally, various cohesions can occur during dry grinding. Therefore,the fact that the cohesion was prevented overall implicates that CNTplays an important role as a grinding aid. However, when viewed onlyfrom the aspect of ultra-fine grinding of aluminum powder, the particlesize decrease obtained from the experiment was not particularlyremarkable.

This is because the present invention focused on elucidating the role ofCNT as a grinding aid or dispersant and understanding dispersionbehaviors of CNT rather than on obtaining ultra-fine particles.

FIGS. 6 a and 6 b are graphs depicting change of density and porosity ofa sintered aluminum compact and a sintered Al-CNT compact. As seen fromFIG. 6, sintered pure aluminum and sintered Al-MWCNT compacts showedchange in density and porosity.

These sintered compacts did not show a significant difference up to 48hours of grinding. At 72 hours of grinding, however, the sinteredaluminum compact showed an abrupt decrease in density. Similarly, bothsintered compacts showed a gradual increase up to 48 hours, but thesintered aluminum compact showed an abrupt increase at 72 hours.

As in the previous results, this also shows that the sintered aluminumcompact believed to suffer from uneven dispersion experiencedsignificant cohesion of powder, which causes the porosity increase andthe density decrease. In contrast, abrupt increase of porosity was notobserved in the Al-CNT composite having even dispersion.

FIG. 7 is a graph depicting hardness change of a sintered aluminumcompact and a sintered Al-CNT compact.

As can be seen from FIG. 7, the two sintered compacts showed similarhardness after up to 12 hours. But, thereafter, the sintered Al-CNTcomposite compact showed a consistent increase of hardness, whereas thesintered aluminum compact showed no further increase.

Increase of hardness in the sintered aluminum compact can be explainedbased on work hardening caused by a significant degree of plasticdeformation during grinding. As a result, the increase of hardnessindicates that a high-quality sintered compact was obtained. Here,improvement of hardness was largely dependent on grinding time, when thesintered compact was prepared by adding CNT to the powder material andgrinding the mixture of the CNT and powder material. For example,hardness increased by about 113% when the sintered compact was preparedfrom powder ground for 72 hours.

INDUSTRIAL APPLICABILITY

The method of producing sintering powder by grinding metal powder alongwith CNT particles according to the present invention can be applied tothe production of high-quality sintered powder materials.

1. A method of producing sintering powder, comprising: adding apredetermined amount of carbon nanotube (CNT) to a raw powder material;and mixing the CNT with the raw powder material and grinding themixture.
 2. The method according to claim 1, further comprising:dispersing the CNT in ethanol before adding the CNT to the raw powdermaterial.
 3. The method according to claim 1, wherein 2 wt % of CNT isadded to 98 wt % of the raw powder material.
 4. The method according toclaim 3, wherein the raw powder material is aluminum powder.
 5. Themethod according to claim 4, wherein the aluminum powder is sintered at550° C.
 6. The method according to claim 1, wherein the CNT is used as agrinding aid during the grinding and the dispersing.
 7. A method ofproducing sintering powder, wherein CNT is used as a grinding aid toreduce only a particle size of powder aluminum without changing particleshape or other characteristics of the powder aluminum.